The Rambam explains in his Peirush Hamishnayos [Keilim 11-6] that פיקה של מתכת is טהורה b/c it doesn't have a שם בפני עצמו but a mere שם לווי.
In the Yad [Keilim 10-13] he offers a different explanation - שמשמשת את העץ.
Why did he change his mind??
[Title of post is pun of the MONTH!!].
